The field of the invention is a cable element and method for making such cable, comprising strands of plastic bearing fibres and a thermoplastic material.
In addition to cables which have been known for a long time and are generally composed of numerous individual steel wires stranded together in a helical manner, plastic cables have recently become more significant since they can be provided with a similar tensile strength to that of wire cables, but have, however, a lower specific density. The strands of such a cable are built in such a way that high-strength synthetic fibres, for example aramide fibres, are at least partially covered with a plastic material so that the individual fibres cannot rub directly against one another, thereby significantly increasing the breaking strength and other characteristics.
DE-AS 12 50 626 discloses a method for producing chords of weldable or sealable plastic, in which a thermoplastic sheet material or a plastifiable coat of lacquer is heated. EP 0 672 781 A1 discloses a method for producing a plastic cable in which the strands are embedded in a plastic material to protect the individual fibres.
U.S. Pat. No. 4,095,404 discloses a method of manufacturing a cable, in which multi-filament yarns are first impregnated with a thermoplastic resin. The yams are first twisted into a helix assembly and then heated so as to fuse the resin coatings of the yarns. Afterwards the heated assembly is cooled so that the coatings of the filaments bond together to form a matrix for the the twisted filaments.
Herein, the individual elements, i.e. the strands, the bearing components of which are synthetic plastic fibres, are twisted in a helical manner, with the fibres being embedded in a plastic material for protection. The problem with regard to the strands is that, on the one hand, they can kink during the stranding process if the individual strands are forced into comparatively narrow radiuses of curvature so that the tensile strength of the cable and other characteristics, such as durability when running over cable pulleys, are no longer guaranteed.
On the other hand, the individual elements do not necessarily assume the helical form which they have to display in the finished cable or cable element, during the stranding process. Rather, they would reassume their straight form if they were to be removed from the cable following the stranding process. For the cable or cable element, this means that the individual elements are exposed to internal stress which leads to the cable or cable element being subjected to a torsional force. Such a torsional force in the cable is undesirable because this will cause the cable to twist. As a consequence, this could result in a shorter service life as well as in differences in length between the individual elements of the cable, for example, between the inner and outer layers of the cable. This is a most undesirable condition. Problems in assembly could also arise.
A certain amount of the internal stress can be reduced in the plastic fibre cable, which has been produced in the normal way, if the finished cable or cable element is stored for a specific period of time (for example one week to one month), during which the plastic material used is able to creep. Alternatively, the finished cable or cable element can be subjected to a heat treatment in a heated oven. However, the problem remains that the individual elements only partially assume the form that they should assume during the production of the cable and up to at least one week thereafter.
Thus, the quality and the load-bearing capacity of a cable produced in this manner can only be assessed following the described period of storage or heat treatment. There is no possibility of intervening in the production process at a point when the characteristics of the finished cable could still be altered to take into consideration requirements.
In a cable according to DE 35 10 808 A1, in which the bearing fibres are coated with a plastic material, the requirement that the individual elements be stress-free is also only achieved by means of the described unsatisfactory measures. This is particularly true in multi-step production methods in which an extrusion process occurs following the stranding of individual strands, by means of which the strands are coated with a plastic material, and subsequently the strands are twisted into a cable which is in turn coated with a suitable material, by extrusion if necessary.
These shortcomings are also true for the cable according to WO 98/16681, in which the individual chords are surrounded by coatings.
It is furthermore known, that a good embedding of strands in the coating of an inner portion of a cable can be achieved by heating the coating material of the inner portion and thus somewhat softening it, and by twisting the strands in this condition. However, not enough heat is released to the strands from the coating in this manner in order for the strands to be twisted in an admissible way without kinks, and so that the strands display the preferred form in the finished cable without internal stress occurring.
An embodiment of the invention provides a method for producing a cable or cable element and a corresponding stranding device by which a cable or cable element, comprising individual elements of plastic bearing fibres and a thermoplastic material which at least partly surrounds the plastic bearing fibres can be produced. The cable is produced according to this method without the storage or heat treatment required according to the prior art, and the characteristics and load-bearing capacity can be controlled during production.
According to an embodiment of the invention, a method for producing a cable or cable element from individual elements is provided. The individual elements comprise strands of a combination of plastic bearing fibres and a thermoplastic material which at least partially surrounds the plastic bearing fibres. The method comprises heating the individual elements before or during a stranding process until they are at least close to plastification, and cooling the individual elements to a solidification temperature of a plastic until they leave a stranding machine.
According to another embodiment of the present invention a cable or cable element comprises individual elements. The individual elements, which are twisted together to form the cable or cable element, comprise strands of plastic bearing fibres and a thermoplastic material which at least partially surrounds the plastic bearing fibres. The individual elements are heated before or during a stranding process until they are at least close to plastification, and are subsequently cooled to a solidification temperature of the plastic until they leave a stranding machine, such that the individual elements are present in the cable or cable element largely without internal stress.
According to yet another embodiment of the present invention, a stranding device for the production of a cable or cable element from individual elements is provided. The individual elements comprise strands of plastic bearing fibres and a thermoplastic material which at least partially surrounds the plastic bearing fibres. The stranding device comprises a heating device which acts on the individual elements and a cooling device located downstream of the heating device in a direction of processing.